So there is that website with Half Life 2 Deathmatch Doom + Quake + Wolf3d (legal)http://www.moddb.com/mods/sda
I have Half Life 2 and Half Life 2 Deathmatch but I have no idea how to put it together. Can anyone help me with that?
I combined two folders but it does not work

Ah, I see what you mean. To install the mod found on that page you just need to unzip the archive and copy the ReWolfenstein3D folder to the ..\Steam\steamapps\SourceMods\ directory. You will also need to have either a Source Engine game installed or Source SDK Base 2007. Once that is done all you should have to do is restart Steam and a link to the mod should appear in the Steam client.